(1) Field of the Invention
This invention relates to a cyclic monoperoxyketal useful as a radical reaction initiator, and more particularly relates to a cyclic monoperoxyketal useful as a polymerization initiator of unsaturated monomers and as a curing agent of unsaturated polyester resin.
(2) Related Art Statement
There are known various type organic peroxides, such as diacyl peroxide, peroxy ester, bisperoxyketal, dialkyl peroxide and the like. These organic peroxides are widely utilized as a polymerization initiator of unsaturated monomers or a curing agent of unsaturated polyester resin at 30.degree.-150.degree. C., or as a crosslinking agent of polyolefin at 150.degree.-200.degree. C. Polymerization initiator or curing agent initiates a radical addition reaction of unsaturated monomers and gives an ordinary polymer or a polymer having a network structure respectively through a chain reaction following to the radical addition reaction. Therefore, the polymerization initiator or curing agent is considered to be one kind of radical reaction initiators. On the contrary, the crosslinking agent generally causes a hydrogen-abstraction reaction from olefin to form a crosslinked polymer. Therefore, the crosslinking agent is different in the reaction mechanism from the polymerization initiator and curing agent, and is generally a very short in the length of radical chain. Recently, the development of highly value-added product having a particular property has been prevailed due to the demand of polymers having high performance and high function. Therefore, radical polymerization initiators having various properties and a high function have been demanded. Such highly value-added product having a particular property is generally produced by the use of various inorganic compounds, various polymer additives and modifiers consisting of organic compounds having a complicated structure. Accordingly, the development of a radical reaction initiator, which is stable in the presence of these additives and modifiers and can exhibit a high activity during the reaction, has been eagerly demanded. Further, even in the field of coating industry, such as paint, adhesive or the like, copolymerization of monomers having various reactive functional groups is carried out, and therefore the development of a polymerization initiator which does not react with these functional groups is demanded. That is, a radical reaction initiator is increasingly used in a composite system.
Among organic peroxides, diacyl peroxide and peroxy ester, both having a carbonyl group, are highly reactive with an acidic or basic inorganic compound, or an organic compound having a functional group, such as hydroxyl group, amino group, sulfide group or the like, and hence the diacyl peroxide and peroxy ester are not preferable in the use in the composite system. Accordingly, bisperoxyketal and dialkyl peroxide, both of which have not a carbonyl group, are preferably used in the composite system. For example, Japanese Patent Application Publication No. 56-29,861 discloses that bis-(t-alkylperoxy)ketal having a neopentyl group is excellent as a radical reaction initiator, and Japanese Patent Application Publication No. 43-1,086 discloses that monoperoxyketal is useful in the crosslinking of a copolymer of ethylene with .alpha.-olefin. Further, U.S. Pat. No. 3,822,317 discloses that 1-methoxy-1-(1,1-dimethylethylperoxy)cyclohexane can be used in the polymerization of styrene.
Bisperoxyketal and dialkyl peroxide are chemically stable, but have a drawback in their high decomposition temperature. Particularly, dialkyl peroxide is generally used at a high temperature of not lower than 120.degree. C., and hence the development of organic peroxide, which has substantially the same decomposition activity as that of peroxy ester at substantially the same low temperature, is demanded. In general, the bisperoxyketal and dialkyl peroxide are apt to cause a hydrogen-abstraction reaction, and are useful as a crosslinking agent. The radical reaction initiator, which is used as a polymerization initiator or curing agent, is generally used for initiating a radical addition reaction, and it is not preferable to cause a large amount of the above described hydrogen-abstraction reaction. For example, in the field of coating industry, there is demanded the development of a polymerization initiator, which is stable in the composite system, and further is small in the side reactions, such as branching, crosslinking and the like, and can introduce effectively a functional group into a polymer. The bisperoxyketal is one kind of bifunctional peroxides, and has a property to widen the molecular weight distribution of the resulting polymer. Further, two t-alkyl peroxy groups bonded to one carbon atom in the bisperoxyketal decrease noticeably the compatibility with unsaturated polyester resin, with the increase of carbon numbers of the bisperoxyketal itself, and results in an incomplete curing very often. Moreover, 1,1-dimethylethylperoxyketal, although it is one of monoperoxyketals, has a very high hydrogen-abstraction property as compared with other monoperoxyketals. Therefore, the use of 1,1-dimethylethylperoxyketal is not preferable as a radical reaction initiator. Therefore, the cyclic monoperoxyketal of the present invention is an improvement of bisperoxyketal and dialkyl peroxide.